Process for coordinated operation of diaphragm and mercury cathode electrolytic cells



May 23, 1967 TADAO UEDA ETAL 3,321,388

PROCESS FOR COOHDINATED OPERATION OF DIAPHRAGM AND MERCURY CATHODE ELECTROLYTIC CELLS Filed Aug. 7, 1963 INVENTORS TAD/4O UFDA BY MOTO/VOBU M/NAGAWA '4 U fiMW ATI'OENEV United States Patent 3,321,388 PROCESS FOR (IQGRDINATED OPERATION OF DIAPHRAGM AND MERCURY CATHGDE ELEC- TROLYTIC CELLS Tadao Ueda and Motonobu Minagawa, Tokyo, Japan, as-

signors to Asahid'enka Kog'yo Kabushilri Keisha, Tokyo, Japan, a corporation of Japan Filed Aug. 7, 1963, Ser. No. 300,519 Claims priority, application Japan, Aug. 9, 1962, 37/ 32,953 8 Claims. (Cl. 264-98) This invention relates to an electrolyzing process for cheaply and efficiently producing an alkali metal hydroxide solution of high concentration containing substantially no alkali metal chloride, chlorine and hydrogen by electrolysis of an alkali metal chloride solution. More particularly, according to the invention a dilute alkali metal hydroxide solution is produced by electrolyzing an alkali metal chloride solution using an improved diaphragm cell in which an ion permselective membrane is provided between an anode and a cathode. The catholyte of the improved diaphragm cell, which contains substantially no alkali metal chloride, is used as a liquid for deamalgamating an alkali metal amalgam produced by electrolyzing an alkali metal chloride using a mercury cathode cell Whose operation is coordinated with said improved diaphragm cell process.

For the electrolytic production of an alkali metal hydroxide solution, it has been suggested to use a mercury cathode cell process and a diaphragm cell process. However, the mercury cathode cell process is not completely satisfactory because the cost of the mercury used is so high that the equipment cost is excessive, the theoretical electrolysis voltage is so high that the cost of electric power per unit weight of the products will be high, and so much of the mercury vapor is mixed in the hydrogen gas produced at the cathode that the hydrogen gas will not be suitable for use in the food industry field such as, for example, in hardening edible oils. Although the cell voltage is low in the known conventional diaphragm cell process, this latter process has disadvantages, for example, that much of the alkali metal chloride will be mixed in the produced alkali metal hydroxide solution, the product will be likely to be colored 'by chemicals produced by the chemical change of the anode material and will be low in purity, and the costs of equipment and evaporation required to concentrate the produced dilute alkali metal hydroxide solution are so high as to increase the cost of production and to reduce the economic value of the diaphragm cell process. Further, as the conventional diaphragm cell process is a low current density electrolysis, the floor space required per unit amount of products will be large.

The present invention is the result of various investigations to eliminate the defects of the conventional mercury cathode cell process and diaphragm cell process. In the present invention, a coordinated electrolytic process employing a mercury cathode cell and an improved diaphragm cell is carried out, as is explained in detail in the following. The advantages of the mercury cathode cell process are retained, namely, that the alkali metal hydroxide will not be colored and will have no alkali metal chloride and that, as a concentrated alkali metal hydroxide so lution is obtained, evaporation will not be required. Also the advantages of the diaphragm cell process are retained, namely, the voltage ,will be low and no mercury will be contained in the produced hydrogen gas. The amount of products per unit floor space required for equipment can be increased greatly so that it is possible to electrolyze alkali metal chloride solutions at low cost and high efliciency.

3,321,388 Patented May 23, 1967 The present invention is characterized by coordinating a conventional mercury cathode cell with an improved diaphragm cell in which an ion permselective membrane is provided as an electrolytic diaphragm between an anode and a cathode. In the present invention, the term improved diaphragm cell is intended to refer to an electrolytic cell which is composed of one electrolyzing unit cell or a plurality of electrolyzing unit cells, each said unit cell having either (1) an anode, a cathode and a cathode compartment separated from an adjoining anode ccmpartment by a cation permselective membrane, or (2) an anode, a cathode and a cathode compartment separated from a center compartment by a cation permselective membrane, the center compartment being separated from an anode compartment by a cation permselective membrane, or (3) an anode, a cathode and a cathode compartment separated from a center compartment by a cation permselective membrane and the center compartment being separated from an anode compartment by an anion permselective membrane, or (4) an anode, a cathode and a cathode compartment separated from a center compartment by a cation permselective membrane and said center compartment being separated from an anode compartment by a liquid permeable electrolytic diaphragm. In such improved diaphragm cells, alkali metal chloride solution is made to flow into the anode compartment and/ or the center compartment, the catholyte containing substantially no alkali metal chloride is fed to a decomposer to deamalgamate, and alkali metal amalgam flowing out of a mercury cathode cell so that a caustic alkali of a high concentration containing substantially no alkali metal chloride, chlorine and hydrogen may be produced.

To explain the present invention more concretely, the improved diaphragm cell used in the present invention includes an improved diaphragm cell comprising a unit cell having an anode, a cathode and one or more cation permselective membranes. The membrane(s) passes substantially no alkali chloride and hardly any hydroxyl ions. The membrane(s) is set between said anode and cathode so as to form either (1) an anode compartment and a cathode compartment, or (2) an anode compartment, 2. center compartment and a cathode compartment. In addition to such cation permselective membrane, an anion permselective membrane can be positioned between the anode and the cation permselective membrane. Further, there can be used a liquid permeable electrolytic diaphragm between the anode and the cation permselective membrane. Also an improved diaphragm cell can be made by combining two or more of the unit cells mentioned above arranged in such order as, for example, the anode-anion permselective membrane-cation permselective mem-brane-cathode-cation permselective membraneanion permselective membrane-anode. In such an improved diaphragm cell, the ion permselective membranes and the electrodes may be arranged so as to be either horizontal or vertical. However, in order to reduce the cell voltage, it is advantageous to arrange them vertically. Further, the anode forming the improved diaphragm cell used in the present invention may be an insoluble anode such as one made of graphite or platinum. However, in the present invention, the improved diaphragm cell is so often an electrolytic cell made by combining many unit cells that it is desirable from the economic point of view to use an anode made by coating titanium or tantalum with platinum. The cathode may be made of such materials as iron, stainless steel, titanium or platinum. The cathode and anode can be made in any form, such as a plate, screen or perforated plate. However, in the present invention, there is no special limitation as to the materials and shapes of these electrodes.

For the cation permselective membrane used in the improved diaphragm cell according to the present invention there can be used any membrane which passes scarcely any hydroxyl iOns, passes substantially no alkali metal chloride which is high in mechanical strength even at high temperatures. For example, a heterogeneous cation permselective membrane can be made by bonding a thermoplastic resin, a cation exchange resin powder or an inorganic ion exchange powder. An interpolymer type cation permselective membrane can be made by melting such high molecular electrolyte as polystyrene sulfonic acid or polymethacrylic acid, together with such synthetic resins as polyacrylonitrile, polyethylene, rubber and others and then molding the melt. Condensed type cation permselective membranes can be made in a suitable fashion such as by condensing phenol sulfonic acid with phenol or formalin. However, the membranes described hereinafter are especially excellent for making the improved diaphragm cell of this invention.

A cation permselective membrane having an organic matrix can be produced by polymerizing at least one of the monovinyl aromatic compound monomers and at least one of polyvinyl aromatic compound monomers in the presence of at least one of the linear polymers. If desired, linear monovinyl compound, a substance not contributing to the polymerization substantially or at all and serving as a plasticizer or an organic solvent, and a reinforcing material of a mineral substance or an organic substance can also be used but their presence is not essential.

Further, a cation permselective membrane can be produced by irradiating organic polymer films such as polyethylene, polypropylene, rubber, ethylene propylene rubber, S.B.R., polyvinylchloride and others with ultraviolet rays or electron rays, mixing at least one of monovinyl aromatic compound monomers, and at least one polyvinyl aromatic compound monomer, with or without at least one linear monovinyl compound monomer and with or without a solvent inducing telomerization such as carbon tetrachloride, carbon tetrabromide, ethylene dichloride, tetrachloroethane and with or without a solvent whose G value is larger than that of the monomer such as benzene, acetone and with or without at least one polyolefin compound monomer. The monomer mixture is absorbed in the form of a liquid or gas into said irradiated polymer film so as to effect graft polymerization and graft telomerization and then introducing cation exchange groups into the film.

Further, a cation permselective membrane can be produced by mixing a monovinyl aromatic compound monomer, a polyvinyl aromatic compound monomer and, as required, any other monomer or solvent and absorbing the monomer mixture into a polymer film, such as polyvinyl chloride film and polymerizing the absorbed monomer mixture and introducing cation exchange groups.

Further, a cation permselective membrane can be produced by polymerizing at least one monovinyl aromatic compound monomer, and at least one of polyvinyl aromatic compound monomers, with or without tetraethyl thiurum disulfide or the like and in the presence or absence of a plasticizer or an organic solvent and with or without a polyolefin compound monomer and a linear monovinyl compound monomer in the form of a membrane by adding, as required, a reinforcing material, then graft polymerizing the obtained film with a monomer such as ethylene, acrylonitrile, vinylchloride, vinylidene chloride, methyl methacrylate by irradiating it with gamma rays or electron rays and introducing cation exchange groups into the membrane. In order that the cation permselective membrane may be adapted to the object of the present invention and may bring about specific desirable results, it must have a high degree of crosslinking, a high mechanical strength, must have high crack resistance at high temperatures and must have the characteristics that it will pass scarcely any hydroxyl ions and will pass substantially no alkali metal chloride. Further, as it can be used under conditions of high temperature, the resistance of the membrane can be lowered so that a reduction of the electrolyzing electric power cost can be achieved.

In producing the cation permselective membrane, the polyvinyl aromatic compound monomer can be divinyl benzene, ar-divinyltoluene, ar-divinylxylene, ar-divinylchlorobenzene, divinylnaphthalene, ar-divinylethylbenzene, trivinylbenzene, and similar polymerizable compounds.

The monovinyl aromatic compound monomer can be styrene, a-methylstyrene, ar-methylstyrene, ar-dimethylstyrene, ar-ethylstyrene, ar-chlorostyrene, vinylnaphthalene ar-sec. butylstyrene, ar-trimethylstyrene, and similar polymerizable compounds.

The monovinyl linear compound monomer can be methylmethacrylate, ethylacrylate, a-chloroethylacrylate, diethyl maleate, vinylchloride, vinylidene chloride, vinylacetate, methylvinylketone, methylvinylether, and similar polymerizable compounds.

The polyolefinic compound monomer can be butadiene, isoprene, 2,3, dimethylbutadiene, 2-chlorobutadiene, bimethallyl, biallyl, divinylether, divinylacetylene, divinylsulfone and similar polymerizable compounds.

The linear polymer can be chlorinated rubber, polyvinylchloride, chlorinated polyisobutylene, polybutadiene, epoxidized polybutadiene, polychloroprene, hypalon, ethylene-propylene rubber, chlorinated isobutene, polymethylmethacrylate, polypropylene oxide, polybutylenoxide and similar non crosslinked polymers.

In the improved diaphragm, the placing an anion permselective membrane between the cation permselective membrane and the anode has the effect of preventing hydrogen ions from passing through the ion permselective membrane and is therefore effective in case the anode chamber liquid is likely to become acidic. However, placing three or more membranes in a unit cell will cause the cell voltage to rise and this is not always advisable. Further, the action of chlorine on the ion permselective membrane can be reduced by placing an electrolytic diaphragm or screen made of a suitable material, such as asbestos or synthetic resin, between the anode and the cation permselective membrane or anion permselective membrane adjacent to the anode.

The mercury cathode cell to be used in coordination with the improved diaphragm cell in the present invention may be a cell of any suitable type, such as a horizontal mercury cathode type, vertical mercury cathode type and gradually sloped mercury cathode type. The decomposer used for decomposing the alkali metal amalgam amalgamated in and flowing out of the mercury cathode cell may be of a horizontal type, tower type and various other types. The deamalgamating substance used may be graphite particles.

The alkali metal chloride solution used as an electrolyte in the present invention may be a solution of sodium chloride, potassium chloride, lithium chloride or it may be sea water, particularly refined concentrated sea water.

The alkali metal hydroxide solution flowing out of the improved diaphragm cell used in the present invention contains substantially no chlorine ions, is at a temperature suitable for contributing to the deamalgamating reaction and can therefore be used as it is as a deamalga-mating solution for the alkali metal amalgam. However, in such case, the higher the concentration of the alkali metal hydroxide solution produced in the cathode compartment, the lower is the efficiency of the cathode current in the above mentioned electrolytic cell. Therefore, it is advantageous to take out the alkali metal hydroxide solution at a concentration of less than about 7 N. 0n the other hand, in case the concentration of the alkali metal hydroxide solution taken out is less than 1.5 N, it will be difiicult to fully show the effect of the present invention. It is therefore desirable to take it out at a concentration higher than that. Further, in order that the concentration of the alkali metal hydroxide solution taken out of the improved diaphragm cell may be adjusted, a proper amount of water or a dilute alkali metal hydroxide solution of less than 2. N may be added to the cathode compartment or the current density may be varied. Thus an alkali metal hydroxide solution of a proper concentration can be obtained.

In carrying out the present invention, the capacity of the mercury cathode cell and that of the above mentioned improved diaphragm cell may be equal to or different from each other. The number of unit cells of both may be equal or difierent. However, in order that the desired concentrated alkali metal hydroxide solution may be obtained at high efiiciency, it is desirable to make the ratio of the capacity of the mercury cathode cell to that of the improved diaphragm cell between 1:2 to :1.

In case the amount of the dilute alkali metal hydroxide solution produced by the improved diaphragm cell for use as a deamalgamating solution is less than the amount needed, another deamalgamating solution may be added as required in addition to said dilute alkali metal hydroxide solution in order to deamalgamate the alkali metal amalgam.

Further, there can be adopted a process wherein the decomposer is formed of two parts, namely, part A in which an alkali metal amalgam is decomposed by adding a deamalgamating solution so as to make an alkali metal hydroxide solution of less than 2 N and part B in which an alkali metal amalgam is decomposed by using an alkali metal hydroxide solution of a concentration of between about 1.5 to 7 N (obtained from the improved diaphragm cell according to the present invention) as a deamalgamating solution so as to make a concentrated alkali metal hydroxide solution. The alkali metal hydroxide solution of less than 2 N obtained from the part A is fed into the cathode compartment of the improved diaphragm cell, is electrolyzed and is taken out as an alkali metal hydroxide solution of a concentration of about 1.5 to 7 N and said alkali metal hydroxide solution is fed into part B so as to deamalgamate the alkali metal amalgam. The advantage of this process is that, if the dilute alkali metal hydroxide solution of less than 2. N, somewhat heated by its deamalgamation of the amalgam, is fed into the cathode compartment of the improved diaphragm cell, it will reduce the cell voltage and will be effective to make the current distribution uniform.

The electrolyte mad-e to flow into the improved diaphragm cell can be circulated as required.

The effect of the coordinated electrolyzing process of the present invention, as has been explained above, can not be achieved with the mercury cathode cell process or the conventional diaphragm cell process alone or even with a coordination of the known conventional diaphragm cell electrolysis and mercury cell electrolysis. Thus, according to the coordinated electrolyzing process of the present invention, as compared with the conventional diaphragm cell process, not only the cost of evaporation of the dilute alkali metal hydroxide will be reduced substantially to zero but also the amount of chlorine ions in the alkali metal hydroxide will become very small, the alkali metal hydroxide product will not be colored at all, and further, a high capacity electrolytic cell of small size can be used so that there is a high degree of utilization of the floor space and automation of the process will be easy. On the other hand, compared with the mercury cathode cell process, the present invention has advantages, such as a reduction of the electrolyzing electric power cost due to the reduction of the voltage of the improved diaphragm cell and a decrease of the amount of mercury used. Further, as no mercury becomes mixed in the hydrogen gas produced from the improved diaphragm cell, it is possible to use the hydrogen for such uses as the processing of foods and the production of edible hardened oils. Further, according to the present invention, the piping arrangements for flowing the alkali metal chloride solution into the mercury cathode cells and improved diaphragm cells and flowing the depleted alkali metal chloride solution can be made in the same system and thereby complicated piping arrangements can be avoided. Further, the alkali metal hydroxide solution produced in the cathode compartment of the improved diaphragm cell will be at a temperature higher than 40 C. because of the Joule heat generated in said electrolytic cell so that it can be directly used'as a deamalgamating solution. Therefore, the present invention has also the advantage that the deamalgamating efiiciency and thermal efficiency are increased.

As described above, the coordinated alkali metal chloride electrolyzing process of the present invention eliminates substantially completely the defects of the conventional diaphragm cell process and mercury cathode cell process but utilizes the merits of both electrolyzing processes to the maximum and makes it possible to electrolyze alkali metal chloride solution at very high efficiency.

In order that the present invention may be more easily understood, the present invention shall now be explained with reference to the accompanying drawings.

FIGURE 1 is a diagrammatic view of an embodiment of the improved diaphragm cell according to the invention.

FIGURE 2 is a diagrammatic view of a diaphragm cell in combination with a mercury cell.

In FIGURE 1, each unit cell consists of an anode 1, a cathode 2, a cation permselective membrane 3, an electrolytic diaphragm 4 and forming an anode compartment 5, a cathode compartment 6 and a center compartment 7. Alkali metal chloride solution is fed from line 3 into the center compartment 7. Chlorine gas is taken out through a chlorine .gas outlet 9 and the depleted alkali metal chloride solution is taken out through a depleted brine outlet 10. The alkali ions in the alkali metal chloride solution reach the cathode 2 through the cation pennselective membrane 3 and become alkali metal hydroxide and hydrogen gas and the hydrogen is taken out through a hydrogen gas outlet 11. The produced dilute alkali metal hydroxide solution is taken out through an alkali metal hydroxide solution outlet 12 and a part of it is circulated again to the cathode compartment. Thus there can be obtained chlorine of a high purity, a dilute alkali metal hydroxide solution containing no colored matter and alkali metal chloride and hydrogen gas containing no mercury.

Referring to FIGURE 2, I is a horizontal mercury cathode cell. II is a decomposing-tower. III is an improved diaphragm cell. The mercury cathode cell I has an anode 13 and a mercury cathode 14. 15 indicates an electrolyzing alkali metal chloride solution. The alkali metal chloride solution is fed into the cell through an alkali metal chloride solution inlet 16. The mixture of the electrolyzed depleted alkali metal chloride solution and the produced chlorine is taken out through a depleted brine outlet 17. The chlorine is separated at a chlorine separating port 18. The mercury is fed into the cell through a mercury inlet 19, flows down while being amalgamated and is fed to the decomposing tower II through the amalgam outlet 20. The improved diaphragm cell 1H comprises a plurality of unit cells each comprising an anode II, a cathode 2, a cation permselective membrane 3 which form an anode compartment 5 and cathode compartment 6. The alkali metal chloride solution is fed through pipe 8 to each anode compartment 5 in the cell and is electrolyzed. The chlorine gas is taken out through a chlorine gas outlet. The depleted alkali metal chloride solution is taken out through the depleted brine outlet. The alkali metal hydroxide solution flows through conduit 24 into the decomposing tower, where it acts as a deamalgamating solution for deamalgamating the alkali metal amalgam fed in from the mercury cathode cell I. It becomes a concentrated alkali metal hydroxide solution and is taken out through a concentrated alkali metal hydroxide solution outlet 21. In such case, in order that the alkali metal amalgam may be more completely deamalgamated, a deamalgamating solution can be fed into the decomposing tower II through the inlet 22 and the produced dilute alkali metal hydroxide solution may be fed by conduit 23 into the cathode compartment of the improved diaphragm cell. The deamalgamated mercury is again fed into the mercury cathode cell.

EXAMPLE 1 The 750 amperes improved diaphragm cell A used in the following Example 2 is composed of 25 electrolyzing unit cells, wherein a cation permselective membrane prepared by the following procedure was fitted between a platinum coated titanium anode and a stainless steel cathode to form the anode compartment and the cathode compartment.

The 1200 amperes mercury cathode cell used in Example 2 and Example 3 has a flowing mercury cathode and graphite anodes and the decomposer used in Example 2 and Example 3 is a tower type decomposer packed with granules as decomposing materials.

The 600 amperes improved diaphragm cell B used in the following Example 3 is composed of 25 electrolyzing unit cells, said unit cell having a graphite anode, a stainless steel cathode and a cathode compartment separated from a center compartment by the cation permselective membrane A and a center compartment separated from an anode compartment by a liquid permeable electrolytic asbestos diaphragm.

Cation. permselective membrane A 10 parts of styrene monomer, 45 parts of divinylbenzene, 40 parts of ethylvinylbenzene, 5 parts of diethylbenzene, 5 parts of polypropylene oxide, 3 parts of chlorinated rubber and 1.5 parts of benzoyl peroxide were mixed together to a homogeneous solution, and the mixture was polymerized into film forms in a polymerization vessel of cm. x 50 cm. x 20 cm. and then strengthened glass plates and polypropylene screens were immersed in the vessel alternately so that each screen was put between two glass plates filled with said monomer solution. The polymerizing vessel was closed and polymerization was carried on at 80 C. for 18 hours under non evaporating conditions. A polymeric film obtained by removing glass plates from the polymer block was leached in ethylene dichloride and plunged into a concentrated sulfuric acid at room temperature to form a cationpermselective membrane A.

Cation permselective membrane B Polyethylene film of 0.1 mm. thickness was immersed to equilibrium at 50 C. in a mixed solution of 80 parts of styrene monomer, 10 parts of ethylvinylbenzene and 10 parts at carbon tetrachloride. The immersed film was irradiated by 'y rays of C0 to a dose of about 8x10 7 at a dose rate of 3i.1 l0 'y/hr. After the film was immersed in benzene to remove retained monomers and homopolymers, it was sulfonated in fuming sulfuric acid at room temperature to form a cation permselective membrane B.

Cation permselective membrane C Cation permselective membrane C was obtained by polymerizing a mixture of 1.5 parts of S-phenylhexene, 40 parts of divinylbenzene, 40 parts of ethylvinylbenzene, 1.5 parts of :butadiene and 1 part of benzoyl peroxide, and sulfonating the resulted film according to the procedure of forming the cation permselective membrane A.

Cation permselective membrane D Mixtures of 40 parts of divinylbenzene, parts of styrene monomer, 40 parts of ethylvinylbenzene and 5 parts of tetraethyl thiurum disulfide was polymerized into a film according to the procedure of forming the cation permselective membrane A. After the film was irradiated by 'y rays in a vacuum to a dose of about 6.7)(10 'y, ethylene monomer was grafted to the film in a reaction vessel filled with ethylene monomer. Cation permselective membrane D was obtained by sulfonating the grafted film with a mixture of one part of fuming sulfuric acid and one part of concentrated sulfuric acid.

EXAMPLE 2 A saturated sodium chloride solution was supplied to the mercury cathode cell described in Example 1 and was electrolyzed with a current density of 50 amperes/dm. and 16.3% of salt depletion rate. The solution was also supplied to the anode compartments of the improved diaphragmcell A described in Example 1 and was elec trolyzed with a current density of 10 amperes/dm. and 15.6% of salt depletion rate. Sodium amalgam of 0.20- 0.28% flowing out of the mercury cathode cell was deamalgamated with 19% sodium hydroxide solution at 6769 C. produced in the cathode compartments of the improved diaphragm cell A. Sodium hydroxide solution of 48-53% was withdrawn from the decomposer de scribed in Example 1. The results obtained are listed in Table 1.

On the other hand, when concentrated sodium chloride solution was electrolyzed, using a conventional diaphragm cell having graphite anodes, a screen cathode made of iron and an asbestos diaphragm, which is known as a Billiter- Siemens cell, and a mercury cathode cell described in Example 1, was sodium amalgam flowing out of the mercury cathode cell was deamalgamated in the decomposer, but sodium chloride deposited in the decomposer and a continuous operation could not be carried out.

TABLE I Improved diaphragm cell used Improved diaphragm cell A Cell B Cation membrane used A B C D A No No Electric power, KWH/t NaOH 3, 080 Evaporation Cost/t NaOH.- 0

EXAMPLE 3 Using the mercury cathode cell described in Example 1, with the improved diaphragm cell B also described in Example 1, saturated sodium chloride solution was supplied to the mercury cathode cell and was electrolyzed with a current density of 50 arnperes/dm. and 18.0% of salt depletion rate, and was also supplied to the center compartrnents and the anode compartments of the improved diaphragm cell B, and was electrolyzed with a current density of 8 amperes/dm. and 15.8% of salt depletion rate. Sodium amalgam of 0.18-0.20% flowing out of the mercury cell was deamalgamated in the decomposer with 15% sodium hydroxide solution at 60 C. produced in the cathode compartments of the improved diaphragm cell B. Sodium hydroxide solution of about 46% was withdrawn from the decomposer. The results are tabulated in Table I.

An improved diaphragm cell composed of 25 unit cells, said unit cell having a platinum coated titanium anode, an iron cathode and a cathode compartment separated from a center compartment by the cation permselective membrane C, and a center compartment separated from an anode compartment by a conventional anion permselective membrane, wherein saturated sodium chloride solution was supplied to anode compartments and center compartments of the improved diaphragm cell, could also be used in process of this invention.

As shown in Table I, the process of this invention has advantages such as the reduction of floor space required per 1,000 amperes of capacity, a decrease of the amount of mercury used to /3 or less, a reduction of electric power cost, and the purity of sodium hydroxide formed is equal to that of mercury cathode cell process. For those reasons, the coordinated process of this invention is far superior to the mercury cathode cell process alone in its economic value.

The coordinated process of this invention has advantages such as a reduction of floor space, an increase in the quality of sodium hydroxide, a reduction of evaporating cost of the dilute sodium hydroxide to substantially zero, as compared with a conventional diaphragm cell process.

Further, in addition to the cation permselective membranes described above, a cation permselective membrane prepared by polymerizing a mixed solution of 2060 parts of mono-vinylaromatic compound monomer, such as styrene, 30-60 parts of polyvinylaromatic compound monomer, such as divinylbenzene, at least one of 05-20 parts of non crosslinked polymers such as chlorinated rubber, chlorinated polyisobutylene, chlorinated polyethylene, 0-20 parts of other materials, such as plasticizers or substances which do not take part in polymerization, 0 20 parts of linear monovinyl compound monomer and sulfonating the resulted polymeric film by a sulfonating agent was also favorable for the improved diaphragm cell. A cation permselective membrane manufactured by irradiating with 'y rays or electron rays such polymeric films as polypropylene, polyethylene, ethylene-propylene rubber, polyblends of ethylene-propylene rubber and polyethlene, and graftpolymerizing to the resulted film the monomer mixture composed of 70 parts of monovinyl aromatic compound monomer, 3-25 parts of polyvinylaromatic compound monomer, 0-10 parts of linear monovinyl compound monomer, and 0.5-20 parts of solvent inducing telomerization, and then introducing cation exchange groups to the grafted film was also favorable for the improved diaphragm cell.

What we claim is:

1. A process for the coordinated operation of diaphragm and mercury cathode electrolytic cells for the electrolysis of alkali metal chloride solutions to produce alkali metal hydroxide solutions, chlorine and hydrogen, the diaphragm cell having at least one cathode compartment separated from an anode compartment by means including a cation permselective membrane, which process comprises: supplying an alkali metal chloride solution both to the mercury cathode cell and to the anode compartment of the diaphragm cell; electrolyzing the solutions flowing through both cells to produce an alkali metal amalgam from the mercury cathode cell and to produce chlorine, hydrogen and a relatively low concentration alkali metal hydroxide solution free of chloride ions from the diaphragm cell;

contacting said alkali metal amalgam with said alkali metal hydroxide solution in a decomposer to deamaglamate the amalgam in order to regenerate the mercury and to form a more concentrated alkali metal hydroxide solution, and separating the mercury from the more concentrated alkali metal hydroxide solution.

2. A process according to claim 1, in which the relatively low concentration alkali metal hydroxide solution has a concentration of between about 1.5 to 7 N and is at a temperature above about 40 C. when it first contacts said amalgam.

3. A process according to claim 1, in which the anode compartment and the cathode are adjacent each other.

4. A process according to claim 1, in which a center compartment is provided between the anode compartment and the cathode compartment and there are cation permselective membranes between the center compartment and both the anode compartment and the cathode compartment.

5. A process according to claim 1, in which a center compartment is provided between the anode compartment and the cathode compartment, there being a cation permselective membrane between the center compartment and the cathode compartment and an anion permselective membrane between the center compartment and the anode compartment.

6. A process according to claim 1, in which a center compartment is provided between the anode compartment and the cathode compartment, there being a cation permselective membrane between the center compartment and the cathode compartment and a liquid permeable diaphragm between the center compartment and the anode compartment.

7. A process according to claim 1, in which pure water is supplied to the cathode compartment.

8. A process according to claim 1, including the further steps of sending a portion of the amalgam to a second decomposer and there contacting it with a deamalgamating solution to make a weak alkali hydroxide solution, and supplying said weak alkali hydroxide solution to the cathode compartment of the diaphragm cell.

References Cited by the Examiner UNITED STATES PATENTS 2,748,072 5/ 1956 Paoloni ct al. 20499 2,967,807 1/1961 Osborne et al. 20498 2,978,401 4/1961 Hoch et a1. 20498 3,051,637 8/1962 Judice et al. 20499 OTHER REFERENCES Chemical Abstracts 50; 11856d (1956). J. Appl. Chem. (U.S.S.R.), 25; 163-7 (1952).

JOHN H. MACK, Primary Examiner. MURRAY TILLMAN, Examiner. L. G. WISE, H. M. FLOURNOY, Assistant Examiners. 

1. A PROCESS FOR THE COORDINATED OPERATION OF DIAPHRAGM AND MERCURY CATHODE ELECTROLYTIC CELLS FOR THE ELECTROLYSIS OF ALKALI METAL CHLORIDE SOLUTIONS TO PRODUCE ALKALI METAL HYDROXIDE SOLUTIONS, CHLORINE AND HYDROGEN, THE DIAPHRAGM CELL HAVING AT LEAST ONE CATHODE COMPARTMENT SEPARATED FROM AN ANODE COMPARTMENT BY MEANS INCLUDING A CATION PERMSELECTIVE MEMBRANE, WHICH PROCESS COMPRISES: SUPPLYING AN ALKALI METAL CHLORIDE SOLUTION BOTH TO THE MERCURY CATHODE CELL AND TO THE ANODE COMPARTMENT OF THE DIAPHRAGM CELL; ELECTROLYZING THE SOLUTIONS FLOWING THROUGH BOTH CELLS TO PRODUCE AN ALKALI METL AMALGAM FROM THE MERCURY CATHODE CELL AND TO PRODUCE CHLORINE, HYDROGEN AND A RELATIVELY LOW CONCENTRATION ALKALI METAL HYDROXIDE SOLUTION FREE OF CHLORIDE IONS FROM THE DIAPHRAGM CELL; CONTACTING SAID ALKALI METAL AMALGAM WITH SAID ALKALI METAL HYDROXIDE SOLUTION IN A DECOMPOSER TO DEAMAGLAMATE THE AMALGAM IN ORDER TO REGENERATE THE MERCURY AND TO FORM A MORE CONCENTRATED ALKALI METAL HYDROXIDE SOLUTION, AND SEPARTING THE MERCURY FROM THE MORE CONCENTRATED ALKALI METAL HYDROXIDE SOLUTION. 